430 ME. r. W. HAEMEE ON THE [Aug. I9OI,. 



oceans. It would not be difficult to suggest such an alteration in the 

 alignment of the isobars as would bring oceanic winds over the deserts 

 of Persia, like those which now deluge Burmah and Bengal with 

 rain during the monsoon.^ It is to such changes (acting locally, 

 I suggest) that the former extension and the subsequent drying-up' 

 of the Pleistocene lakes of Utah and Nevada were due. 



The succession of events in the region in question, according to 

 Prof. Gilbert'^ and Mr. Eussell,^ was as follows : — a jDcriod, previous 

 to the flooding of the plains, with a climate as rainless as that of 

 the present day ; afterwards a moist period, during which Lake 

 Lahontan attained a depth of 500 feet, and Lake Bonneville a level 

 1000 feet higher than that of the Great Salt Lake at present ; 

 then a time of desiccation ; followed by a return to humid 

 conditions, and finally, by the evaporation of the water to its 

 present level. 



During the pre-Lahontan and inter-Lahontan periods, with their 

 arid climate, the distribution of pressure may have been, more or less, 

 similar to that of the present day (figs. 17 & 19, pp. 450 & 454). 

 If, however, the anticylones of the North Pacific and of the North 

 American continent had been shifted from their present positions to 

 those shown in fig. 20 (p. 456), a low-pressure system obtaining 

 statistically at the same time ofi" the western coast of Mexico, similar 

 to that which I suppose may have existed in the Mediterranean area 

 during the pluvial period of the Sahara, more humid conditions 

 would have been caused over the southern and western portions of the 

 United States by warm and moist southerly or south-easterly winds 

 from the Gulf of California, or the Gulf of Mexico. At the present 

 day, a comparatively moist climate in Florida and the regions ad- 

 joining it is caused by such winds, but their influence does not 

 extend far north-westward. 



These changes in the basin of Nevada, assuming their origin to 

 have been meteorological, may have been due to the presence, at 

 the pluvial stages of the period in question, of an ice-sheet in Noi^h 

 America, the pressure of the anticyclone of which disturbed the 

 atmospheric conditions which had existed in the pre-Lahontan 

 period, altering the prevalent direction of the winds, those con- 

 ditions being re-established when, owing to the disappearance or 

 retreat of the ice, that pressure ceased.^ 



^ The failure of the monsoon rains to which the last Indian famine was due 

 coincided with an increased rainfall in Mauritius, the Seychelles, Zanzibar, and 

 part of South Africa, An alteration in the relative positions of the centres 

 of high and low pressure diverted at that time the oceanic winds from their 

 usual course, causing them to enrich some regions with moisture_^at the expense 

 of others. 



•' 'Lake Bonneville ' Snd Ann. Rep. U.S. Geol. Surv. (1881) pp. 172-76.^ 



=^ 'Lake Lahontan ' 3rd Ann. Rep. U.S. Geol. Surv. (1882) p. 230. 



* M]'. Russell not only expresses the Oj^inion generally held by American 

 geologists, that the existence of these lakes was contemporaneous with the Glacial 

 Period, but also that the fluctuations in their water-level indicate changes of 

 climate, the humid periods having coincided with increased cold, and the dry 

 periods with comparative warmth, op. cit. p. 231. 



